This invention relates generally to methods for coating a moving web and, more particularly, slide bead coating methods, and most particularly to slide bead coating methods for manufacturing photographic film and paper products.
Bead coating is well known in the prior art as described, for example, in U.S. Pat. No. 2,761,791 to Russell. Bead coating is used to apply multiple layers of liquid to a moving substrate. In the method typically referred to as slide bead coating, a multilayer composite comprised of superimposed individual liquid layers is delivered to the moving substrate through the use of a coating die. At the end of the coating die, the layers form a continuous liquid bridge or coating bead between the die and the moving substrate. The slide bead coating method is useful for making thin, highly uniform, composite elements suitable for numerous applications, including photographic, thermographic, x-ray, and photoelectric films, among others.
In the practice of slide bead coating, the lowermost layer typically has a wet thickness of approximately 40-100 microns and a viscosity of 3-10 cp. as noted in U.S. Pat. No. 4,001,024 to Dittman. Alternatively, Dittman teaches the use of a lowermost layer having a wet thickness of only 2-12 xcexcm and a viscosity of 1-8 cp. This thin lowermost layer is known in the art as a carrier layer. Because a carrier layer is thin, dryer load is reduced and operation at high substrate speeds is possible. One drawback to the use of a carrier layer in slide bead coating is the formation of mixing artifacts between the carrier layer and adjacent upper layer as described in Dittman.
Subsequent improvements to the carrier layer method of slide bead coating have also been directed toward dryer efficiency. Some of these improvements describe the use of layers having high viscosity. Because high viscosity layers generally have a higher percentage of solid material, there is less water to be removed during drying. U.S. Pat. No. 4,113,903 to Choinski, for example, teaches the use of a pseudoplastic carrier layer having a high viscosity in the range of 20-200 cp. Although a pseudoplastic carrier layer has a high viscosity at low shear, the liquid obtains substantially lower viscosity at the high shear rates present in the coating bead. Moreover, the use of a high viscosity liquid in the lowermost layer is claimed to overcome the mixing artifacts noted by U.S. Pat. No. 4,001,024 to Dittman, between the carrier layer and adjacent upper layer. However, the practical significance of a pseudoplastic carrier layer is diminished by the fact that special additives may be required to achieve high viscosity and pseudoplastic behavior. Such additives may be expensive or incompatible with the function of the finished film.
Similarly, U.S. Pat. No. 4,572,849 to Koepke, suggests the use of high viscosity fluids in the uppermost layers to reduce dryer load. These high viscosity layers are used in conjunction with the carrier layer described previously by U.S. Pat. No. 4,001,024 to Dittman. As is the case for a high viscosity carrier layer, a high viscosity liquid in the uppermost layers may have a correspondingly high solids concentration. As a result, the time and energy required to remove water during the drying process may be minimized.
U.S. Pat. No. 4,863,765 to Ishizuka, describes a carrier layer of pure water at 40xc2x0 C. having a wet thickness of less than 2 xcexcm. Although an ultra thin aqueous carrier layer reduces dryer load, the improvement over U.S. Pat. No. 4,001,024 to Dittman is relatively small. One drawback to the use of water as a carrier layer is that water is incompatible with many organic coating fluids. Incompatible coating fluids create precipitate on the surface of the coating die, and ultimately produce large amounts of waste due to streaks. Another drawback to U.S. Pat. No. 4,863,765 to Ishizuka, is that the carrier layer temperature is high at 40xc2x0 C. When applying layers with highly volatile organic solvents, high temperatures are undesirable since premature evaporation of solvent causes precipitate or a crust to form on the coating die resulting in streak non-uniformity in the film. Application of layers at high temperatures may also cause undesirable mottle patterns to form as a result of rapid flashing of solvent during drying. Both streak and mottle non-uniformities result in unacceptable levels of waste.
Although the carrier layer methods described above allow for a modest increase in substrate speed by reducing dryer load, substrate speed is ultimately limited by other factors when drying capacity is adequate. For example, the formation of regularly spaced streaks has been noted by U.S. Pat. No. 4,863,765 (Ishizuka) when substrate speeds are high. Other coating artifacts are created as air becomes entrained in the coating bead at very high substrate speeds. These defects are conspicuous at the point of application of the coating to the moving substrate as well as during subsequent visual inspection of dried film samples. Dried samples have numerous small bubbles and localized thickness variations when air is entrained at the coating bead. Defects resulting from entrained air are generally observed with all coating methods including the carrier layer methods described above.
Another coating artifact encountered with the carrier layer method is the formation of streaks caused by deposition of coating materials on the surface of the coating die. Coating materials from an upper layer may be deposited on a slide surface during start-up procedures as well as during coating when the upper slots of the die are serviced to clear bubbles or slugs. Once deposited on the slide surface, these materials typically obstruct the flow of coating fluids on the slide surface for several minutes and produce undesirable streaks in the final film. For the carrier layer method, contamination is especially problematic for the first slide surface which typically has only a thin layer of fluid between the first slide surface and the upper layers. Although the contamination is eventually washed away, in some cases contamination may persist for several minutes. Even at a moderate line speed of 200 cm/s, a single contamination episode of only three minutes would generate 360 linear meters of streak waste.
U.S. Pat. No. 5,861,195 to Bhave describes contamination of the surfaces of a coating die caused by the application of layers having incompatible materials. In particular, when a polymer in the lowermost layer is not compatible with a polymer in the uppermost layers, undesirable contamination of the slide surface may produce streak waste. This particular slide contamination results from the precipitation of incompatible polymers and is called strikethrough. U.S. Pat. No. 5,861,195 to Bhave teaches the use of a slightly higher density fluid as a carrier layer to minimize contamination episodes. Examples describe the addition of polymer to the lowermost layer to increase density. Another more elaborate remedy involves separating the lowermost layer from the incompatible materials of the uppermost layers by a second layer having both a higher density than the uppermost layers and a polymer compatible with the lowermost layer. However, adjustment of fluid density requires the use of additional materials which may be expensive or harmful to the performance of the final product. Moreover, the use of additional layers to protect the lowermost carrier layer is complicated and may have undesirable effects on drying efficiency as well as on product performance.
It is therefore an object of the present invention to overcome limitations inherent to slide bead methods when applying organic coatings at high substrate speeds.
It is a further object of the present invention to minimize streak artifacts created by contamination of the surface of the coating die.
Yet another object of the present invention is to provide a slide bead coating method which reduces the duration of slide contamination and streak formation.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by forming a multilayer composite on a slide surface of a coating hopper, the multilayer composite including a carrier layer that is an organic solvent or blend of organic solvents that are substantially free of other constituents, the carrier layer having a viscosity of less than 1 cp and a wet thickness of not more than 5 xcexcm; flowing the multilayer composite down the slide surface and over a coating lip of the coating hopper; and forming a coating bead between the coating lip and the web. In the practice of the method of the present invention it is preferred that the web be travelling at a speed of at least about 50 cm/sec. These conditions are readily attained at the temperatures routinely used to prepare films with highly volatile organic liquids. In particular, the use of the method of the present invention is shown to substantially extend substrate speed by elimination of defects caused by entrained air in the coating bead. Moreover, the use of an organic solvent or an organic solvent blend is found to overcome streak artifacts created when aqueous liquids are used as carrier layers in conjunction with layers containing organic solvents. The present invention also overcomes undesirable mixing artifacts between the lowermost and upper layers since the carrier layer is only a vehicle and is not part of the finished film. In addition, the use of the method of the present invention can be employed to substantially minimize the duration of slide contamination and streak formation through the selection of an appropriate organic solvent or blend of organic solvents as a carrier layer. Therefore, the present invention provides an advantageous method for the fabrication of thin, uniform films such as required for photographic elements or other similar elements.
In the practice of the method of the present invention, one or more of the upper layers preferably has a viscosity that is greater than the viscosity of the carrier layer. When the carrier layer is formed of a blend of organic solvents, preferably at least two of the following components: methanol, ethanol, isopropanol, n-propanol, n-butanol, acetone, methylethyl ketone, methylisobutyl ketone, toluene and methylene chloride are contained in the blend. It is also preferred that the layers above the carrier layer contain at least one of the following polymeric materials: cellulosics, polyvinylbutyrals, polyurethanes, polycarbonates and polyesters. Further in the practice of the method of the present invention it is preferred that the layers above the carrier layer have a combined wet thickness in the range of range of 1-500 xcexcm and at least one of such upper layers has a viscosity in the range of 100-5,000 cp. In addition, one or more of such upper layers may include water.
Although the present invention is discussed herein with particular reference to a slide bead coating operation, those skilled in the art will understand that the present invention can be advantageously practiced with other multilayer coating operations. For example, high substrate speeds should be achievable with multilayer extrusion hopper coating operations and multilayer curtain coating operations. Practical applications of the present invention include photographic, thermographic and x-ray films as well as photographic, thermographic and ink jet papers among others.